This is the first application filed for the present invention.
Not Applicable.
The present invention relates to converters for converting alternating current (AC) power main voltage to a voltage suitable for driving a lamp.
Most electronic converters for converting AC power main voltage to a voltage for driving a lamp, such as a halogen lamp, are based on self-oscillating technology using bipolar transistors. Since bipolar transistors are current operating devices, obtaining feedback for oscillation is relatively simple. However, bipolar transistor converters suffer from several disadvantages. For example they are subject to secondary breakdown phenomena, increased current leakage and increased power losses at elevated temperatures. The practical limit for junction temperature is 100xc2x0 C. (case temperature typically 85xc2x0 C.). Bipolar transistor converters are also expensive for high voltage applications (for example 277V, 240V and 220V). They also are less efficient in operation than field-effect transistors, because a typical limitation on frequency of operation is 35 kHz. Protection against fault conditions is difficult in a simple circuit using bipolar transistors. In addition, size reduction is limited due to operating frequency limitations, and it is difficult to achieve UL Class B temperature classification (135xc2x0 C. maximum insulation limitation) without a sacrifice in reliability.
U.S. Pat. No. 6,157,551 to Barak, et al., assigned to Lightech Electronic Industries Ltd., which issued Dec. 5, 2000, teaches a power converter using bipolar transistors. However, this converter suffers from the foregoing disadvantages.
U.S. Pat. No. 6,208,086 to Nerone, assigned to General Electric, which issued Mar. 21, 2001, teaches a power converter using N-channel and P-channel field effect transistors (FETs). Nerone achieves size reduction and improves efficiency by operating at higher frequencies (30 kHz-90 kHz). However, Nerone fails to address the issue of high temperature operation and fault protection. Besides, P-channel FETs are expensive compared to N-channel FETs.
There therefore exists a need for a converter that is simple and inexpensive to construct, while providing fault protection and achieving reliable, sustained operation at elevated operating temperatures.
The present invention provides a converter for converting alternating current (AC) power main voltage to a voltage suitable for driving a lamp. The converter comprises a rectifier circuit connectable to the AC power main, adapted to rectify the AC power main voltage and adapted to provide a direct current (DC) voltage; a driver circuit adapted to receive the DC voltage from the rectifier circuit, and provide a driver output voltage and a driver output current, and further adapted to receive an output current limiting signal; a starter circuit for providing a starter signal that initiates oscillation at an operating frequency in the driver circuit; a sensing circuit for sensing the driver output current and providing the output current limiting signal in response to the sensed driver output current; and a transformer for transforming the driver output voltage to a voltage suitable for driving a lamp such as a halogen lamp.
The sensing circuit may be further adapted to provide overheating protection for the converter. Overheating protection can be provisioned in a plurality of ways. In one embodiment, the sensing circuit includes a Negative Temperature Coefficient (NTC) thermistor that is in good thermal contact with the converter. A resistance of the NTC thermistor is reduced as a temperature of the converter rises. This causes the output current limiting signal to reduce output current from the driver circuit when the converter overheats. The reduction in driver output current permits the converter to cool and inhibits component failure. In another embodiment, a silicon diode is used rather than a NIC thermistor. A switching threshold of the silicon diode is reduced as a temperature of the converter rises. This causes the output current limiting signal to reduce output current from the driver circuit to halt the rise in temperature.
In accordance with another aspect of the invention, a method is provided for controlling an output voltage of a driver circuit in response to an output current of a converter for converting an AC (alternating current) power main voltage to a voltage suitable for driving a lamp. The method comprises the steps of sensing the converter output current; testing whether the sensed converter output current exceeds a threshold; sensing the extent to which the converter output current exceeds the threshold; triggering a latch when the sensed converter output current exceeds the threshold and stopping an oscillation of the driver circuit; re-setting the latch after a period of time related to an extent to which the converter output current exceeds the threshold, and re-starting the oscillation of the driver circuit.
Advantages of the invention include power savings, extended service life for converter components, reduced power loss, and reduced heat generation.
A further advantage of the invention is an avoidance of high cost electrolytic or tantalum capacitors, and improved reliability at high temperature operation.
Another advantage of the invention is a protection against fault conditions, such as output short circuits.
A further advantage of the invention is an extended operational temperature range for the converter, which enables the converter to achieve an Underwriters Laboratories (UL) Class B temperature classification up to 135xc2x0 C., which is a maximum insulation limitation.
Yet another advantage of the invention is providing a converter with an operating frequency that is greater than 43 kHz, which enables smaller converter packages and more power efficient converters.
Still another advantage of the invention relates to decreased current leakage and switching losses at elevated temperature resulting from the use of field-effect transistors for switching drive current.
The invention also provides a converter, that is reliable, versatile, compact and efficient, with a reduced parts count.